Electrostatic chucks, also known as susceptors, are employed to support various substrates, such as wafers, during manufacture of semiconductor devices. Wafers are secured to a chucking surface by an electrostatic force generated between an external electrode and an electrode embedded in a dielectric chuck body. A heating element also can be embedded in the chuck body.
The electrode and the optional embedded heating element are connected to a power source by electrical contacts, or terminals. However, limitations in the methods of manufacturing electrostatic chucks and, frequently, other design factors, require that the electrode and/or heating element be manufactured separately from the contact. For example, the electrode and/or heating element sometimes is prefabricated, and the terminal, or terminals, must be bonded to it. In addition, the dielectric material of the chuck body typically is formed by firing a green form. The dimensions of green form change during firing, often necessitating that the electrical contacts be attached after formation of the chuck body. A common method for forming a joint layer between the electrode and a connector is by brazing. Another method, employed post-sintering for forming an electrical contact, includes a nickel-molybdenum eutectic composition.
One disadvantage in the use of eutectic compositions as contacts is their tendency to form intermetallic compounds. Since generally such intermetallic compounds are brittle, they can cause the electrical contact to fracture, thereby causing the chuck to fail.
Therefore, a need exists for an electrostatic chuck, and a method for fabricating an electrostatic chuck, that minimizes or overcomes the above-referenced problems.
It has been found that Moxe2x80x94Ni contacts having low Ni contents are less likely to fracture during use. Without wishing to be tied to a theory, it is believed that conventional contacts made from intermetallic compositions often fail in use because of the brittle nature of intermetallics. It is believed that the absence of intermetallic species in the novel contacts of the present invention minimized or eliminated this failure mode, thereby producing a valuable commercial product having a relatively high resitance to fracture.
Some alloys encompassed within the scope of the contact of the present invention have been found to have coefficients of thermal expansion (CTE""s) significantly different from that of the conventional chuck body (typically AlN). In this art, it is preferred that the CTE of the alloy be within about 10% of the CTE of the chuck body. Thus, there is a desire for contacts which have not only no intermetallics but also the appropriate CTE. The present inventors have found that adding certain xe2x80x9cCTE-adjusting compounds,xe2x80x9d such as, for example, tantalum, to the contact, not only adjust the CTE appropriately, but also does not produce intermetallic species.
The present invention is directed to an electrostatic chuck and to a method for fabricating an electrostatic chuck. The invention also generally is directed to a susceptor, such as an electrostatic chuck.
The electrostatic chuck includes a chuck body and an embedded electrode. A electrical contact extends from the electrode. At least one of either the electrode or the electrical contact includes an alloy of a first metal and a second metal and both the electrode and the electrical contact include the first metal. In one embodiment of the invention, the contact does not include intermetallic compounds. In another embodiment, essentially all of the second metal is dissolved in the first metal.
The electrostatic chuck of the invention can further include a metallic heating element embedded in the chuck body. The optional heating element can include a first metal. An electrical contact, including an alloy having a metal component common to the metallic heating element, the alloy consisting essentially of a solution of metal elements.
In still another embodiment, a susceptor of the invention includes a ceramic body and a metallic element embedded in the ceramic body. At least one electrical contact extends from the metallic element. At least one of either the metallic element or the electrical contact includes an alloy of a first metal and a second metal and both the metallic element and the electrical contact include the first metal. In one embodiment of the invention, the contact does not include intermetallic compounds. In another embodiment, essentially all of the second metal is dissolved in the first metal. Examples of metallic elements include but are not limited to electrodes or heating elements.
In a preferred embodiment, the chuck or susceptor ceramic body is fabricated from aluminum nitride. The first metal can include, for example, molybdenum, tungsten or combinations thereof. In addition to the first metal, the electrical contact includes a second metal, such as, for example, nickel, cobalt or combinations thereof. The amount of the second metal present in the contact essentially prevents intermetallic compounds from forming between the first metal and the second metal.
The method of the invention includes the steps of molding a first portion of a ceramic precursor, forming a recess in the first portion of the ceramic precursor and depositing an electrical contact or an electrical contact precursor in the recess. In one embodiment, the electrical contact or electrical contact precursor includes a first metal and a second metal. The method also includes the steps of depositing an electrode or an electrode precursor onto the first portion of the ceramic precursor, whereby the electrode or electrode precursor overlays the electrical contact or electrical contact precursor, and molding a second portion of a ceramic precursor onto the electrode or electrode precursor. The method further includes heating the ceramic precursor, thereby forming the electrostatic chuck. In one embodiment of the invention, the ceramic precursor is a green form of aluminum nitride. In a preferred embodiment the same ceramic material is employed to form the first and second portions.
The method of the invention also includes forming a chuck body with an embedded electrode, forming an opening to expose a portion of the electrode, depositing an electrical contact precursor at the exposed portion of the electrode and heating the electrical contact precursor, thereby forming an electrostatic chuck. In one embodiment, the electrode includes an alloy of a first metal and a second metal and the electrical contact precursor includes the first metal. In another embodiment, the electrical contact precursor includes an alloy of the first metal and the second metal and the electrode includes the first metal. In still another embodiment, both the electrode and the electrical contact include an alloy of the first and the second metal. In yet another embodiment, both the electrode and the electrical contact precursor include the same alloy composition.
The method of the invention can further include the step of heating the first portion of the ceramic precursor, prior to depositing the electrode or electrode precursor, to form a first ceramic body.
This invention has many advantages. For example, the electrostatic chuck of the invention has low impedance and good mechanical adherence of the electrical contact to the electrode. By providing a junction wherein the second metal essentially is dissolved completely in the first metal, formation of brittle intermetallic compounds essentially is prevented or minimized, thereby resulting in an electrostatic chuck, or susceptor, of relatively high reliability and robustness.